1. Technical Field
The present invention relates to a refrigerant compressor, and more particularly, to a slant plate type compressor, such as a wobble plate type compressor, having a variable displacement mechanism which is suitable for use in an automobile air conditioning system.
2. Description of the Prior Art
Slant plate type piston compressors including variable displacement or capacity adjusting mechanisms for controlling the compression ratio of a compressor in response to demand are generally known in the art. For example, Japanese Utility Model Application Publication No. 63-134181 discloses a wobble plate type compressor including a cam rotor driving device and a wobble plate linked to a plurality of pistons. Rotation of the cam rotor driving device causes the wobble plate to nutate and thereby successively reciprocate the pistons in the corresponding cylinders. The stroke length of the pistons and thus the capacity of the compressor may be easily changed by adjusting the slant angle of the wobble plate. The slant angle is changed in response to the pressure differential between the suction chamber and the crank chamber.
In the above-mentioned Japanese Utility Model Application Publication, the crank chamber and the suction chamber are linked in fluid communication by a first path or passageway. A valve mechanism is disposed in the first passageway in order to control fluid communication between the crank and suction chambers by the opening and closing of the first passageway. The valve mechanism generally includes a solenoid, a plunger and a valve member disposed on one end of the plugner. The solenoid receives two external signals, one of which represents the heat load on an evaporator of a cooling circuit and another which represents the amount of demand for accelerating an automobile.
The solenoid induces various electromagnetic forces in response to changes in the two external signals and thereby changes the axial position of the plunger so that the first passageway is opened and closed by the valve member. Hence, the angular position of the wobble plate is varied in a range from the maximum to the minimum slant angles responsive to changes in the two external signals such that the capacity displacement of the compressor is thereby adjusted and the suction chamber pressure is maintained at a predetermined constant value.
The compressor further includes a second passageway, separate from the first passageway, and communicating the crank chamber with the suction chamber. A safety valve device including a ball member and a coil spring elastically supporting the ball member is disposed in the second passageway. The safety valve device opens and closes the second passageway in response to changes in the pressure differential between the crank chamber and the suction chamber. The second passageway is opened when the pressure differential between the crank chamber and the suction chamber exceeds a predetermined value. Therefore, when communication between the crank chamber and the suction chamber is blocked for a long time period of time due to trouble in the valve mechanism, thereby causing an abnormal rise in the crank chamber pressure because of blow-by gas leaking past the pistons in the cylinders as the pistons reciprocate, the second passageway is opened so as to forcibly and quickly reduce the crank chamber pressure and thereby prevent an abnormal pressure differential between the crank and suction chambers. As a result, excessive friction between the internal component parts of the compressor caused by the abnormal differential between the crank chamber and the suction chamber can be prevented.
In this prior art embodiment, however, the second passageway is separate from the first passageway such that the process of forming the second passageway and the process of disposing the safety valve device in the second passageway are additional steps required during the manufacturing of the compressor. Accordingly, the manufacturing process of the compressor is complicated by this requirement.
Therefore, a strong need exists for a compressor having a variable displacement control mechanism which can be easily manufactured and which can prevent an abnormal pressure differential between the crank chamber and the suction chamber.